Olefin-based resins, such as polyethylene, polypropylene and polybutene-1, have ease of manufacturing, are inexpensive and are excellent in lightweight properties, moldability, insulating properties, water resistance, chemical resistance and mechanical strength, and therefore, they have been used in an extremely wide variety of fields.
Specifically, these olefin-based resins have been used in almost all the surrounding fields, such as fields of various packaging films, electric wire coating materials, pipes, sheets, bottles, automotive interior or exterior trim and general merchandise.
In the industrial world, recently, weight lightening of members has been required from the viewpoints of energy saving and ecology, and foamed products of various materials and foaming techniques have been paid attention. As for the foamed products, not only weight lightening but also impartation of functions, such as cushioning properties, heat insulation/heat retaining properties and sound absorption properties, can be achieved by combining resin functions with porous structure (foaming), so that because of their high functional characteristics, adoption of the foamed products has been expected in a wide variety of fields.
In the field of foaming, however, the olefin-based resins have been only used limitedly as compared with urethane and polystyrene. This is caused by the influence of difficult processing in various molding processes because of low melt viscosity. Therefore, it has become an industrial problem to increase melt viscosity of the olefin-based resins during molding, specifically, melt tension thereof.
For example, in patent literatures 1 to 4, ethylene/α-olefin/non-conjugated polyene copolymers having a long-chain branched structure have been disclosed. However, when these copolymers are added to olefin-based resins, such as polyethylene, polypropylene and polybutene, phase separation takes place during molding, and a clear sea-island structure is exhibited. Therefore, the melt tension hardly increases as the whole system. On that account, the effect of improving processability in extrusion molding, blow molding or the like is not found so greatly, and there is room for improvement particularly in expansion ratio and cell uniformity in the foam molding.
Further, a method to increase melt tension by adding ultrahigh molecular weight compounds has been attempted, but this method has problems that the production process is complicated, it can be utilized limitedly only in a specific molding process because the fluidity is lowered, the melt tension improvement effect is not found so greatly, etc.
On the other hand, a method to increase melt tension by partially crosslinking molecular chains using electron rays or a peroxide and thereby increasing long-chain branched structures has been also attempted. In α-olefin-based copolymers such as polypropylene, however, the main chain scission reaction by radicals precedes the crosslinking reaction rate, and therefore, it is necessary to use a polyfunctional monomer as a crosslinking aid. In this case, it is difficult to control a molecular weight or the degree of branching, and it is difficult to obtain desired properties. Further, addition of a polyfunctional monomer brings about problems, such as bleed out of a low molecular weight component, lowering of weathering resistance and lowering of mechanical strength. Moreover, such a crosslinked resin is liable to form a gel component in the thermoforming process, and this is a cause of bad appearance such as fish eye during inflation molding or sheet forming.
In a patent literature 5, there is description of a syndiotactic propylene-based copolymer composed of propylene/α-olefin/non-conjugated polyene and/or conjugated polyene, but this copolymer does not have a long-chain branched structure, and still has a problem of moldability.